1. Field of the Invention
The present invention relates to a band gap reference voltage circuit which generates a reference voltage.
2. Description of the Related Art
A conventional band gap reference voltage circuit is now described. FIG. 5 is a circuit diagram illustrating the conventional band gap reference voltage circuit.
As temperature increases, a base-emitter voltage Vbe1 of an NPN bipolar transistor 101 decreases with a negative temperature coefficient. In this occasion, because an NPN bipolar transistor 102 is larger in emitter area than the NPN bipolar transistor 101, a base-emitter voltage Vbe2 of the NPN bipolar transistor 102 decreases with a negative temperature coefficient to be lower than the base-emitter voltage Vbe1 of the NPN bipolar transistor 101.
Here, an amplifier 106 operates so that a node A and a node B have the same voltage, and hence a voltage (ΔVbe=Vbe1−Vbe2) determined by subtracting the base-emitter voltage Vbe2 from the base-emitter voltage Vbe1 is generated across a resistor 105. It is found from the expression above that the voltage ΔVbe has a positive temperature coefficient. Accordingly, a current I2 flowing through a resistor 104 and the resistor 105 also has a positive temperature coefficient, and a voltage generated across the resistor 104 also has a positive temperature coefficient. Variation of the voltages generated across the resistor 104 and the resistor 105 each of which has a positive temperature coefficient and variation of the base-emitter voltage Vbe2 having a negative temperature coefficient cancel each other. Therefore, a reference voltage Vref does not depend on temperature, irrespective of a temperature coefficient of a current I1 flowing through a resistor 103 (see, for example, JP 2003-258105 A).
However, if a power supply voltage Vdd varies, due to a gate-source or gate-drain parasitic capacitance of a transistor (not shown) which is provided at an input stage of the amplifier 106, a gate voltage of the transistor also varies. This causes variation of the voltages at the nodes A and B. As a result, the voltage ΔVbe inevitably depends on the variation of the power supply voltage Vdd, which deteriorates a power supply rejection ratio of the band gap reference voltage circuit.